We report thin-film InAs/GaAs QD solar cells

fabricated by epitaxial lift-off of 3-inch wafers containing

QD epi-structures with high in-plane QD density. External

quantum efficiency measurements demonstrate enhanced QD

harvesting in the thin-film configuration. Numerical simulations

show that remarkably high increase of the QD photocurrent

may be achieved by replacing the planar rear mirror with

micro-structured photonic gratings. Measurements of diffraction

efficiency of grating prototypes realized on GaAs wafers by

nanoimprint lithography are presented

Aho, Timo

Bauhuis, G.

Bissels, Günther

Cappelluti, Federica

Cédola, A. P.

Elsehrawy, FARID KHALED MOHAMED FARID

Guina, Mircea

Khalili, A.

Kim, D.

Liu, H.

Mulder, Peter

Niemi, Tapio

Schermer, John

Wu, J.

English

We report thin-film InAs/GaAs QD solar cells
fabricated by epitaxial lift-off of 3-inch wafers containing
QD epi-structures with high in-plane QD density. External
quantum efficiency measurements demonstrate enhanced QD
harvesting in the thin-film configuration. Numerical simulations
show that remarkably high increase of the QD photocurrent
may be achieved by replacing the planar rear mirror with
micro-structured photonic gratings. Measurements of diffraction
efficiency of grating prototypes realized on GaAs wafers by
nanoimprint lithography are presented

Federica Cappelluti

A. P. Cédola

A. Khalili

Farid Elsehrawy

G. Bauhuis

Peter Mulder

John Schermer

Günther Bissels

Timo Aho

Tapio Niemi

Mircea Guina

D. Kim

J. Wu

H. Liu

PORTO@iris (Publications Open Repository TOrino - Politecnico di Torino)

04 August 2020POLITECNICO DI TORINORepository ISTITUZIONALEEnabling high-efficiency InAs/GaAs Quantum Dot Solar Cells by Epitaxial Lift-Off and Light Management / Cappelluti,Federica; Cédola, A. P.; Khalili, A.; Elsehrawy, FARID KHALED MOHAMED FARID; Bauhuis, G.; Mulder, Peter;Schermer, John; Bissels, Günther; Aho, Timo; Niemi, Tapio; Guina, Mircea; Kim, D.; Wu, J.; Liu, H.. - ELETTRONICO. -(2018). ((Intervento presentato al convegno IEEE 44th Photovoltaic Specialists Conference (PVSC) tenutosi aWashington DC nel June 2017.OriginalEnabling high-efficiency InAs/GaAs Quantum Dot Solar Cells by Epitaxial Lift-Off and Light ManagementieeePublisher:PublishedDOI:Terms of use:openAccessPublisher copyrightcopyright 20xx IEEE. Personal use of this material is permitted. Permission from IEEE must be obtained for all otheruses, in any current or future media, including reprinting/republishing this material for advertising or promotionalpurposes, creating .(Article begins on next page)This article is made available under terms and conditions as specified in the  corresponding bibliographic description inthe repositoryAvailability:This version is available at: 11583/2726878 since: 2019-03-01T23:10:06ZIEEEEnabling High-Efficiency InAs/GaAs Quantum Dot Solar Cells byEpitaxial Lift-Off and Light ManagementF. Cappelluti∗, A. P. Ce´dola∗, A. Khalili∗, Farid Elsehrawy∗, G. Bauhuis†, P. Mulder†, J. Schermer†,G. Bissels‡, T. Aho§, T. Niemi§, M. Guina§, D. Kim¶, J. Wu¶, H. Liu¶∗Department of Electronics and Telecommunications, Politecnico di Torino, Italy†Institute for Molecules and Materials, Radboud University, Nijmegen, The Netherlands‡tf2 devices B.V., Nijmegen, The Netherlands§Optoelectronics Research Centre, Tampere University of Technology, Tampere, Finland¶Department of Electronic and Electrical Engineering, University College London, London, United KingdomAbstract—We report thin-film InAs/GaAs QD solar cellsfabricated by epitaxial lift-off of 3-inch wafers containingQD epi-structures with high in-plane QD density. Externalquantum efficiency measurements demonstrate enhanced QDharvesting in the thin-film configuration. Numerical simulationsshow that remarkably high increase of the QD photocurrentmay be achieved by replacing the planar rear mirror withmicro-structured photonic gratings. Measurements of diffractionefficiency of grating prototypes realized on GaAs wafers bynanoimprint lithography are presented.Index Terms—solar cell, epitaxial lift-off, thin-film, quantumdot, light trappingI. INTRODUCTIONQuantum dots are very attractive band-engineered materialsfor the development of high-efficiency single-junction andmulti-junction solar cells. Studies of quantum dot solar cells(QDSCs) were motivated by the idea of taking advantageof sub-bandgap transitions to absorb otherwise wasted lowenergy photons and improve short circuit current [1]. Ifproperly engineered, QDSCs can be exploited to realize theintermediate band operation [2]. Despite their great potential,the improvement in conversion efficiency, if any, with QDsis still marginal. The most challenging issue to demonstratehigh-efficiency QDSCs is achieving at the same time a largeincrease of the short circuit current (Jsc) and preservationof high open circuit voltage (Voc). In QDSCs, regardlessof their operating regime (thermally-limited or intermediateband), increasing the number of QD layers or the QD density,besides being technologically challenging, causes a larger Vocpenalty. Thus, a trade-off exists between maximizing the Jscand conserving the Voc. On the other hand, implementinglight-trapping schemes to strongly enhance the QD photonharvesting can yield efficiency comparable to or higher thanthe one of single-junction cells even for thermally-limitedQDSCs [3], and enable the demonstration of QDSCs oper-ating in the intermediate band regime [4].In this work, we report our recent results in the devel-opment of high-efficiency thin-film GaAs cells integratingQDs and photonic nanostructures. These devices can be fab-ricated using cost-effective and scalable fabrication processessuch as Epitaxial Lift-Off [5] for the thin-film processingand NanoImprint Lithography (NIL) to pattern even sub-wavelength period gratings over large areas [6].Fig. 1. Cross-section of the analysed QD-based solar cell and calculatedenergy band diagram at short-circuit condition under AM1.5G illumination.II. THEORETICAL BACKGROUND AND EXPERIMENTThe photovoltaic performance of InAs/GaAs QDSCs arestudied within a drift-diffusion framework accounting for thepeculiar carrier transfer dynamics between QD states andtransport bands [7]. Such cells work in the thermally limitedoperation, i.e. the second photon absorption is negligiblewith respect to the thermal escape of carriers from QDstates to transport bands, and the Voc is limited by theradiative recombination through the QD ground-state [7].Such a penalty, which indicatively ranges between 50-250 mVdepending on the QD size and effective Ground State (GS)bandgap [8], [7], [9], can be substantially reduced by partiallycharging the QD states and inhibiting the carrier captureand subsequent recombination through the GS [10], [11],[12]. A further advantage of QD charging is the passivationof defects induced by the strained epitaxy, thus limitingthe Voc degradation due to non radiative recombination inreal QD solar cells [12]. By combining QD charging andlight-trapping, QDSCs with efficiency higher than their bulkcounterpart are theoretically feasible [3], [13].In this work, we adopt a device design based on a deepjunction structure with lightly doped emitter and thin base, asshown in Fig.1. The cell behavior is studied for different pho-tonic configurations: a) wafer-based cell (single-pass); b) thin-film cell with planar metallic reflector at the bottom (double-pass); c) thin-film cell with light-trapping scheme yielding inFig. 2. Efficiency of the QD cell vs. QD doping density for different photonicconfigurations (see text). The horizontal lines show the efficiency of thereference single-junction cells.the weak absorption limit an optical path enhancement of 2n2,n being the GaAs refractive index [3]. QDs are described as athree level system characterized by energy levels identified bythe measured photoluminescence spectra (GS bandgap = 1.17eV) and radiative lifetime of 1 ns [7], [12]. Doping-dependentSRH recombination lifetime of minority carriers is set to 500ns in the emitter and 20 ns in the base. Fig. 2 shows anoverview of the impact of the various photonic configurationsand of QD doping (implemented through direct doping) onthe achievable efficiency of the QD cells. The efficiency ofthe corresponding single-junction GaAs cells is also shown.A significant efficiency increase is achieved in both single-junction and QD thin-film cells with respect to their wafercounterpart. Moreover, in the light-trapping configuration, anabsolute improvement of about 0.7% is calculated for the thin-film QD cell with optimum doping level (around 14 e/dot)with respect to the thin-film single junction cell.The fabricated cells include high in-plane density (over 8×1010 cm−2, see Fig. 3) QD layers fabricated through the Sb-mediated QD growth technique [14]. Three 3-inch wafers, onefor wafer-based processing of the single-junction baseline celland two for ELO thin-film processing of single-junction andQD solar cells were grown. 3-inch films were released byELO and processed into cells (area=0.25 cm2) with planargold mirror on the backside, as shown in Fig. 4. No ARCwas used.III. RESULTS AND DISCUSSIONMeasured current-voltage characteristics are reported inFig. 5. The thin-film single-junction and QD cells show com-parable Voc, with a reduction with respect to the wafer-basedcell which is attributed to lattice defects propagating from thesubstrate to the epilayers. The dark current of all the thin-film cells is in fact dominated by the presence of a diode-likeFig. 3. AFM image of one stack of InAs/GaAs QDs with high in-planedensity and photoluminescence spectra at T=300 K and different excitationpowers.Fig. 4. Single-junction GaAs (left) and QD InAs/GaAs (right) thin-film cellsfrom epitaxial lift-off of 3-inch diameter epi-structures.Fig. 5. Current-Voltage characteristics of wafer-based regular GaAs cell andthin-film GaAs and QD cells measured under AM1.5G illumination.Fig. 6. Measured and simulated EQE spectra of thin-film single-junction GaAs solar cells (left) and thin-film QD solar cells (right).shunt defect, with reverse saturation current density of about4 nA/cm2 and ideality factor of 2.External Quantum Efficiency (EQE) spectra of the thin-filmcells are reported in Fig. 6, showing the typical extension ofthe EQE in the long wavelength range due to the insertion ofQDs: for λ > 880 nm, the wafer QDSC - not shown here -provided a short-circuit current of about 0.4 mA/cm2, whichincreases to 0.6 mA/cm2 in the thin-film QDSC.By moving to a micro-structured grating realized on thebottom of the cell, the QD absorbance can be enhanced moresignificantly. In particular, we are studying diffraction gratingconfigurations where light-trapping is pursued by couplinglight into high order diffraction modes propagating outside ofthe cell escape cone. These gratings require a period lengthlarger than the incident wavelength. Several designs werestudied through simulations based on the Rigorous Coupled-Wave Analysis (RCWA) method. The QD photogenerated cur-rent was studied as a function of the grating period and height.From the results, the relevant parameter for light-trappingoptimization turned to be the aspect ratio (height/period),whose optimum ranges between 0.32 and 0.36 for gratingperiods between 2-3 µm [15]. By exploiting optimized gratingconfigurations, the simulated absorbance spectra (Fig. 7) showa remarkable increase at the GaAs band edge and in theQD range. With pyramidal gratings, an enhancement of QDphotocurrent of about 13 times with respect to the wafer-basedconfiguration is predicted.Prototype gratings (Fig. 8(a)) were fabricated on a GaAswafer and diffraction efficiency measured by the scanningangle detector technique (Fig. 8(b)). A comparison betweenmeasured and simulated diffraction efficiency of one pro-totype is reported in Fig. 8(c)-(d) showing good agreement.Significant power coupling to the first two diffraction ordersis found. Further optimization of the grating can be carriedFig. 7. Calculated absorbance spectra for different photonic configurations.The dashed line indicates the classical Yablonovitch limit [16].out in order to better match the diffraction spectrum to theQD absorption spectrum. Experimental demonstration of lighttrapping enhancement by integration of a rear diffractiongrating in the thin-film QDSC is ongoing.IV. ACKNOWLEDGEMENTSThis project has received funding from the EuropeanUnion’s Horizon 2020 research and innovation programmeunder grant agreement No 687253, www.tfqd.eu.REFERENCES[1] V. Aroutiounian, S. Petrosyan, A. Khachatryan, and K. Touryan,“Quantum dot solar cells,” JAP, vol. 89, no. 4, pp. 2268–2271, 2001.[2] A. Luque and A. Martı´, “Increasing the efficiency of ideal solar cellsby photon induced transitions at intermediate levels,” Phys. Rev. Lett.,vol. 78, no. 26, pp. 5014–5017, 1997.Fig. 8. Profilometer image of back grating of 3 µm period fabricated by NIL (a) and sketch of the prototype samples realized on GaAs wafer (b). Measured(c) and simulated (d) diffraction efficiency.[3] F. Cappelluti, M. Gioannini, G. Ghione, and A. Khalili, “Numericalstudy of thin-film quantum-dot solar cells combining selective dopingand light-trapping approaches,” in 2016 IEEE 43rd Photovoltaic Spe-cialists Conference (PVSC), June 2016, pp. 1282–1286.[4] A. Mellor, A. Luque, I. Tobı´as, and A. Martı´, “The feasibility of high-efficiency inas/gaas quantum dot intermediate band solar cells,” SolarEnergy Materials and Solar Cells, vol. 130, pp. 225–233, 2014.[5] G. Bauhuis, P. Mulder, E. Haverkamp, J. Huijben, and J. Schermer,“26.1% thin-film gaas solar cell using epitaxial lift-off,” Solar EnergyMaterials and Solar Cells, vol. 93, no. 9, pp. 1488–1491, 2009.[6] J. Tommila, V. Poloja¨rvi, A. Aho, A. Tukiainen, J. Viheria¨la¨, J. Salmi,A. Schramm, J. Kontio, A. Turtiainen, T. Niemi et al., “Nanostructuredbroadband antireflection coatings on alinp fabricated by nanoimprintlithography,” Solar Energy Materials and Solar Cells, vol. 94, no. 10,pp. 1845 – 1848, 2010.[7] M. Gioannini, A. Cedola, N. Di Santo, F. Bertazzi, and F. Cappelluti,“Simulation of quantum dot solar cells including carrier intersubbanddynamics and transport,” IEEE J. Photovoltaics, vol. 3, no. 4, pp. 1271–1278, Oct 2013.[8] G. Jolley, L. Fu, H. Lu, H. H. Tan, and C. Jagadish, “The role of inter-subband optical transitions on the electrical properties of ingaas/gaasquantum dot solar cells,” Prog. Photovolt: Res. Appl., 2012.[9] C. G. Bailey, D. V. Forbes, R. P. Raffaelle, and S. M. 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Enabling high-efficiency InAs/GaAs Quantum Dot Solar Cells by Epitaxial Lift-Off and Light Management

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Enabling high-efficiency InAs/GaAs Quantum Dot Solar Cells by Epitaxial Lift-Off and Light Management

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